**Core Concept**
The question pertains to Chronic Lymphocytic Leukemia (CLL) and Small Lymphocytic Lymphoma (SLL), both of which are types of non-Hodgkin lymphoma characterized by the clonal expansion of mature-appearing lymphocytes. CLL/SLL involves the accumulation of malignant B cells in the bone marrow, lymphoid organs, and peripheral blood.

**Why the Correct Answer is Right**
CLL/SLL is typically a disease of older adults, with most cases diagnosed in individuals over the age of 60. The pathogenesis of CLL/SLL involves the accumulation of mutations in the immunoglobulin genes, leading to the activation of the B cell receptor signaling pathway. This results in the continuous proliferation of malignant B cells, which are often characterized by the expression of CD5 and CD23 antigens. The correct answer will be a statement that does not accurately reflect the typical presentation or pathophysiology of CLL/SLL.

**Why Each Wrong Option is Incorrect**
**Option A:** CLL/SLL typically presents with a slow progression of symptoms, but it can be aggressive in some cases. This statement is generally true, so it is not the correct answer.
**Option B:** CLL/SLL often involves the accumulation of malignant B cells in the bone marrow, lymphoid organs, and peripheral blood. This statement is also true, so it is not the correct answer.
**Option C:** CLL/SLL is often associated with the expression of CD5 and CD23 antigens on the surface of malignant B cells. This statement is true, so it is not the correct answer.
**Option D:** CLL/SLL is typically a disease of younger adults, with most cases diagnosed in individuals under the age of 40. This statement is false, as CLL/SLL is typically a disease of older adults.

**Clinical Pearl / High-Yield Fact**
CLL/SLL is often associated with the accumulation of mutations in the immunoglobulin genes, which can lead to the activation of the B cell receptor signaling pathway. This results in the continuous proliferation of malignant B cells, which can be targeted with specific therapies such as ibrutinib and venetoclax.

**Correct Answer: D. CLL/SLL is typically a disease of younger adults, with most cases diagnosed in individuals under the age of 40.**